The following is a guest blog post by Dr. Susan Molchan, one of our independent expert editorial contributors.
Last week PLOS Medicine published the results of a large observational study that showed an association between the selective serotonin reuptake inhibitor (SSRI) class of anti-depressants (e.g. Prozac and Paxil) and violent crime in young people. As a psychiatrist, I read the paper with interest, along with associated reports in the media. In a research news story in The BMJ (subscription required), a favorite source of news and analysis, I found trouble right in the first sentence: “Use of selective serotonin reuptake inhibitors (SSRIs) increases the rate of violent crime among young adults . . . “
Oh no, I thought: they used an active verb to describe an association — a classic error and source of confusion when reporting on the results of observational studies. Such studies explore correlations between two elements, and by design, cannot say whether one thing caused another. In the offending first sentence, the verb “increases” indicates that the drugs caused the increase in crime rate.
The PLOS study authors clearly state their results show “associations between SSRIs and violent crime.” (My emphasis added.) In the discussion section of the paper, they note, “The reported association between SSRIs and violent crime in young people cannot be interpreted causally . . .” An Editors’ Summary following the PLOS article similarly cautions that the findings “cannot, however, prove that taking SSRIs actually causes an increase in violent crime . . . “. None of these restraining comments made their way into The BMJ‘s coverage.
Such infractions of language are serious, in that they are a big reason why scientific findings are not communicated accurately or effectively in the media. When the media jump to causal conclusions based on studies that can’t support such statements, they project a false certainty that is liable to be punctured by the next study that comes along. This leads people to throw up their hands and say things like, “Just wait a year or two and scientists will be saying just the opposite.” It undermines trust in science, as well as the media.
HealthNewsReview.org features a nice primer on this whole association versus causation business, with several excellent examples of how science can get lost in translation by a misplaced word or two in media reports.
As a starting point, let’s all agree to “just say no” when it comes to active verbs and associations.
So how do we tell what causes what? In a nutshell, the only way to establish whether a drug causes fewer arrests for violent crime is to do an experiment, a randomized clinical trial. One group of people would be randomized to take the drugs, and the other group a placebo, with their violent crimes recorded over time. Of course with a question like the one at hand, this is never going to happen. People are prescribed SSRIs for a reason, and withholding effective treatment for some wouldn’t be ethical. Many questions simply can’t be answered by using clinical trials.
Observational studies such as the one published in PLOS have become increasingly sophisticated, and over time add to an evidence base that can help establish cause and effect. This happened with the association between smoking and lung cancer; the associations in early studies were strong; the more one smoked, the more the risk of cancer. Over time, all properly performed studies (not interfered with by tobacco companies) consistently and strongly pointed to the same conclusion.
Unlike the dose-related risk of cancer with smoking, however, those taking higher doses of SSRI drugs did not show an association with increased risk of violence in this study. As per the paper: “ . . . the increased hazard for violent crime conviction in younger people was not found in individuals with therapeutic SSRI exposures.”
So perhaps those patients whose illness was being adequately treated did not show the increased risk for violence, despite taking SSRIs? This could change the whole message people may get about the study’s results from, “Don’t use these drugs because they are dangerous,” to “When used appropriately, the SSRIs are safe and are not associated with risk of violence in younger people.”
The BMJ story alluded to this interesting conundrum, saying that the association with violent crime applied only at lower doses of the SSRIs. And it quotes one of the study authors, Dr. Fazel, who noted that young people “may be less likely to adhere to treatment and may also have more residual symptoms, such as impulsivity and hostility, that are risk factors for violence and that may not be controlled with lower doses.”
While that’s commendable context, the qualification is buried in the very last paragraphs of the story. I think it’s likely to be overwhelmed by the introductory sentence about SSRIs “increasing” risk.
Just to show that we’re not picking on The BMJ, I’ll add that a story in the LA Times also tripped up in the first sentence: “Researchers have identified a troubling side effect of a widely prescribed class of antidepressants—they may make some patients more likely to commit violent crimes.” The active verb here: “make.”
But the story recovered nicely with this caveat about the results: “The study doesn’t prove that the SSRIs were responsible for the observed increase in criminal violence among teens and young adults, the researchers said. However, it does add to evidence that ‘the adolescent brain may be particularly sensitive to pharmacological interference,’ they wrote.”
None of this is to say that SSRIs may not predispose some people for some period of time in their treatment to agitated, aggressive behavior that sometimes leads to violence. Accumulating evidence, including from randomized trials, suggests that this could be a real issue for some young people as the PLOS study authors acknowledge.
But that’s not what the story being reported on found. It will take more research to determine whether these associations represent a true cause-and-effect link. No association between drugs and human behavior is simple.
Editor’s note: We’ve written about problematic BMJ news releases on observational studies in the past (as well as occasions when they got it right). Past examples:
But the story discussed above is a different animal — a BMJ Research News story. We’ll keep an eye on these types of stories and hold them the same high standard we maintain for other BMJ communications.